Digital documents, containing a mixture of text and pictures are proliferating. Similarly, the number of digital output devices such as digital printers are increasing. A vast majority of digital printers can print only in a bilevel mode, either ink is printed or no ink is printed. The task of converting digital documents into digital output on bilevel output devices involves a process known as halftoning. In halftoning the original continuous tone input is converted to a bilevel image. Digital halftoning is used for printing a picture (or more generally displaying a picture on some two-dimensional medium). It uses small dots with a limited number of colors such that it appears to consist of many colors when viewed at a proper distance. For example, a picture printed with black and white dots may appear to contain various shades of gray when viewed at some distance.
Commonly used methods for digital halftoning include dithering algorithms which use threshold arrays (also called dither matrices) or dither masks. A fundamental problem in halftoning is that text is generally not rendered well. Processing pictures as text or text as pictures results in poor print output quality. An example of this poor quality would be text areas erroneously printed as halftones. This results in the text looking blurred rather than sharp. It is desirable to have an algorithm that can print both picture and text areas well.
A possible solution is to segment the text and picture areas and process them separately. The composite document containing text and pictures is segmented into text and picture areas. Only the picture areas are halftoned. This technique is time consuming and error-prone in so much that the segmentation is rarely performed with total accuracy. Furthermore, when there are text areas within pictures the segmentation is not well defined.
Goertzel and Thompson (U.S. Pat. No. 4,654,721) present a technique for edge enhancement in halftones which assigns a number of pixels to print in an area and orders the pixel values in a larger area to determine where the printed pels should be placed. The Goertzel-Thompson invention is based on lower resolution input data and is not capable of reproducing the level of image detail required by many applications.